Lateral phase change memory

ABSTRACT

A lateral phase change cell may be formed over a semiconductor substrate. The lateral cell, in some embodiments, may be exposed to light so that the same cell may be addressed by both optical and electrical signals.

BACKGROUND

This invention relates generally to phase change memories.

Phase change memories use phase change materials, i.e., materials thatmay be electrically switched between a generally amorphous and agenerally crystalline state, as an electronic memory. One type of memoryelement utilizes a phase change material that may be, in oneapplication, electrically switched between generally amorphous andgenerally crystalline local orders or between detectable states of localorder across the entire spectrum between completely amorphous andcompletely crystalline states.

Typical materials suitable for such an application include variouschalcogenide elements. The state of the phase change materials is alsonon-volatile. When the memory is set in either a crystalline,semi-crystalline, amorphous, or semi-amorphous state representing aresistance value, that value is retained until reprogrammed, even ifpower is removed. This is because the program value represents a phaseor physical state of the material (e.g., crystalline or amorphous).

Thus, there is a need for different ways to make phase change memories.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a greatly enlarged cross-sectional view of one embodiment ofthe present invention;

FIG. 2 is a greatly enlarged cross-sectional view at an early stage ofmanufacture in accordance with one embodiment of the present invention;

FIG. 3 is a greatly enlarged cross-sectional view at a subsequent stageof manufacture in accordance with one embodiment of the presentinvention;

FIG. 4 is a greatly enlarged cross-sectional view at a subsequent stageof manufacture in accordance with one embodiment of the presentinvention;

FIG. 5 is a greatly enlarged cross-sectional view at a subsequent stageof manufacture in accordance with one embodiment of the presentinvention;

FIG. 6 is a greatly enlarged cross-sectional view at a subsequent stageof manufacture in accordance with one embodiment of the presentinvention;

FIG. 7 is a greatly enlarged cross-sectional view at a subsequent stageof manufacture in accordance with one embodiment of the presentinvention;

FIG. 8 is a greatly enlarged cross-sectional view at a subsequent stageof manufacture in accordance with one embodiment of the presentinvention;

FIG. 9 is a greatly enlarged cross-sectional view at a subsequent stageof manufacture in accordance with one embodiment of the presentinvention;

FIG. 10 is a greatly enlarged cross-sectional view at a subsequent stageof manufacture in accordance with one embodiment of the presentinvention;

FIG. 11 is a greatly enlarged cross-sectional view at a subsequent stageof manufacture in accordance with one embodiment of the presentinvention; and

FIG. 12 is a system schematic depiction of one embodiment of the presentinvention.

DETAILED DESCRIPTION

Referring to FIG. 1, a pair of side-by-side cells 10 are formed over asubstrate 26, in one embodiment of the present invention. Each cell 10includes a phase change material 36 between a pair of laterallydisplaced resistive electrodes 34 on either side of the phase changematerial 36.

For example, a top electrode 40 may be coupled to a column line and anelectrode 34 may be coupled to a row conductive line 44. Of course, thedesignations “row” and “column” may be reversed and are merely used forconvenience.

In one embodiment, the cells 10 may be on the same conductive line 44,each cell 10 selectively coupled to the line 44 by a selection device42. The selection devices 42 may be, for example, field effecttransistors, diodes, or a chalcogenide alloy rich in Selenium as threeexamples. The selection devices 42 and the conductive line 44 may beformed in a substrate 26 in one embodiment of the present invention.

The selection device 42 may be coupled to a bottom electrode 24. Thebottom electrodes 24 of the cells 10 may be separated by an insulator30. On opposed sides of the insulator 30 is a liner 20. The liner 20electrically couples the bottom electrode 24 to a resistive electrode 34associated with each cell 10.

Outbound of each electrode 34 is a phase change material 36 that mayextend vertically and may be described as L-shaped in one embodiment.Outbound of the phase change material 36 is an opposed electrode 34.Thus, the phase change cells 10 are arranged laterally so that theelectrodes 34 are spaced apart in a horizontal direction, sandwichingintermediate phase change material 36.

The cells 10 may be formed within a dielectric layer 22 and anotherdielectric layer 30. A light transmissive material 38 such as adielectric glass or a high bandgap and electrically insulating material38, such as arsenic sulfide (As₂S₃), may also be positioned between therespective cells 10. In some embodiments of the present invention, eachcell 10 may be electrically addressed for programming, reading, orwriting. In one embodiment, the cells 10 may be optically addressed, twoat a time, by a light signal, as indicated, to provide an extradimension of cell 10 access.

Referring to FIG. 2, a pair of spaced bottom electrodes 24 may bedefined over a substrate 26. The bottom electrodes 24 may be formed ofaluminum as one example. The bottom electrodes 24 may be shaped orisolated.

A layer 22 of dielectric material may be formed over the substrate 26 onthe bottom electrode 24, followed by an etch stop layer 25, followed byan additional layer 22 a. A first trench 46 may be formed centrallytherethrough. In one embodiment, the layers 22, 22 a may be oxide.

As shown in FIG. 3, the conductive liner 20 may be deposited into thefirst trench 46 and over the layer 22 a. In one embodiment, the liner 20may be tungsten or another strong conductor. As shown in FIG. 4, theportion of the liner 20 at the bottom of the first trench 46, as well asa portion overlying the layer 22 a, may be cleared down to the etch stoplayer 25, for example, using directional etching techniques.

Turning next to FIG. 5, the structure of FIG. 4 may be covered by aninsulator 30 that may be deposited into the first trench 46 and over theinsulator 22. The insulator 30 may be, for example, silicon nitride. Theinsulator 30 may act as a subsequent chemical mechanical polishing stopin some embodiments.

Next, as shown in FIG. 6, a second trench 38 may be formed using anetchant that does not attack the liner 20 using a mask 32. Then, asshown in FIG. 7, a resistive electrode 34 may be deposited, for example,using chemical vapor deposition. In one embodiment, the resistiveelectrode 34 may include titanium silicon nitride. As shown in FIG. 8,the resistive electrode 34 may be cleared at the bottom of the secondtrench 38 and atop of the island 39 and over the mask 32.

Thereafter, the phase change material 36 may be deposited by a chemicalvapor deposition process so that a uniform conformal growth occurs onthe side of electrode 34. An in situ backsputter may be utilized toremove a central region of that material 36 so that electrical contactbetween the two cells is broken. An L-shaped phase change material 36results. Then the remaining structure may be filled with a chalcogenidelight transmissive material 38, such as As₂S₃, in one embodiment of thepresent invention. As shown in FIG. 11, this process may be followed bya chemical mechanical polishing that stops on the layer 30. Thereafter,as shown in FIG. 1, the electrodes 40 may be patterned and deposited.

In one embodiment, the phase change material 36 may be a non-volatile,phase change material. A phase change material may be a material havingelectrical properties (e.g., resistance) that may be changed through theapplication of energy such as, for example, heat, light, voltagepotential, or electrical current.

Examples of phase change materials may include a chalcogenide materialor an ovonic material. An ovonic material may be a material thatundergoes electronic or structural changes and acts as a semiconductoronce subjected to application of a voltage potential, electricalcurrent, light, heat, etc. A chalcogenide material may be a materialthat includes at least one element from column VI of the periodic tableor may be a material that includes one or more of the chalcogenelements, e.g., any of the elements of tellurium, sulfur, or selenium.Ovonic and chalcogenide materials may be non-volatile memory materialsthat may be used to store information.

In one embodiment, the phase change material 36 may be chalcogenideelement composition from the class of tellurium-germanium-antimony(Te_(x)Ge_(y)Sb_(z)) material or a GeSbTe alloy, although the scope ofthe present invention is not limited to just these materials.

The phase change material 36 may be programmed into one of at least twomemory states by applying an electrical signal to the material 36. Anelectrical signal may alter the phase of the material 36 between asubstantially crystalline state and a substantially amorphous state,wherein the electrical resistance of the material 36 in thesubstantially amorphous state is greater than the resistance of thematerial 36 in the substantially crystalline state. Accordingly, in thisembodiment, the material 36 may be adapted to be altered to one of atleast two resistance values within a range of resistance values toprovide single bit or multi-bit storage of information.

Programming of the material 36 to alter the state or phase of thematerial may be accomplished by applying voltage potentials to the lines40 and 44, thereby generating a voltage potential across the material36. An electrical current may flow through a portion of the material 36in response to the applied voltage potentials, and may result in heatingof the material 36.

This heating and subsequent cooling may alter the memory state or phaseof the material 36. Altering the phase or state of the material 36 mayalter an electrical characteristic of the material 36. For example, theresistance of the material 36 may be altered by altering the phase ofthe material 36. The material 36 may also be referred to as aprogrammable resistive material or simply a programmable material.

In one embodiment, a voltage potential difference of about 3 volts maybe applied across a portion of the material 36 by applying about 3 voltsto a lower line 44 and about zero volts to an upper line 40. A currentflowing through the material 36 in response to the applied voltagepotentials may result in heating of the material. This heating andsubsequent cooling may alter the memory state or phase of the material.

In a “reset” state, the material 36 may be in an amorphous orsemi-amorphous state and in a “set” state, the material may be in acrystalline or semi-crystalline state. The resistance of the material inthe amorphous or semi-amorphous state may be greater than the resistanceof the material in the crystalline or semi-crystalline state. Theassociation of reset and set with amorphous and crystalline states,respectively, is a convention. Other conventions may be adopted.

Due to electrical current, the material 36 may be heated to a relativelyhigher temperature to amorphisize memory material and “reset” memorymaterial (e.g., program memory material to a logic “0” value). Heatingthe volume of material to a relatively lower crystallization temperaturemay crystallize memory material and “set” memory material (e.g., programmemory material to a logic “1” value). Various resistances of material36 may be achieved to store information by varying the amount of currentflow and duration through the volume of material.

The information stored in material 36 may be read by measuring theresistance of the material 36. As an example, a read current may beprovided to the material 36 using opposed lines 40, 44 and a resultingread voltage across the memory material may be compared against areference voltage using, for example, a sense amplifier (not shown). Theread voltage may be proportional to the resistance exhibited by thememory storage element. Thus, a higher voltage may indicate thatmaterial is in a relatively higher resistance state, e.g., a “reset”state. A lower voltage may indicate that the material is in a relativelylower resistance state, e.g., a “set” state. A current based system mayalso be used.

In some embodiments of the present invention, a lateral plate design mayhave symmetry in all directions leading to an approximately rectangularchalcogenide volume. Thus, such a design may give uniformity to theelectric field, current flow, power dissipation and heat flow in someembodiments. Because of the design, the total volume of phase changematerial 36 may change phase, promoting tight resistance states from oneprogramming event to another, reducing erratic programming, loss ofdynamic range, resistance or endurance issues, or possible alloy changesphase during programming event in some embodiments.

Turning to FIG. 12, a portion of a system 500, in accordance with anembodiment of the present invention, may be used in wireless devicessuch as, for example, a personal digital assistant (PDA), a laptop orportable computer with wireless capability, a web tablet, a wirelesstelephone, a pager, an instant messaging device, a digital music player,a digital camera, or other devices that may be adapted to transmitand/or receive information wirelessly. The system 500 may, for example,be used in any of the following systems: a wireless local area network(WLAN) system, a wireless personal area network (WPAN) system, or acellular network, although the scope of the present invention is notlimited in this respect.

The system 500 may include a controller 510, an input/output (I/O)device 520 (e.g., a keypad display), a memory 530, and a wirelessinterface 540 coupled to each other via a bus 550. It should be notedthat the scope of the present invention is not limited to embodimentshaving any or all of these components.

The controller 510 may comprises, for example, one or moremicroprocessors, digital signal processors, microcontrollers, or thelike. The memory 530 may be used to store messages transmitted to or bythe system. The memory 530 may also be optionally used to storeinstructions that are executed by the controller 510. During theoperation of the system 500 it may be used to store user data. Thememory 530 may be provided by one or more different types of memory. Forexample, a memory 530 may comprise a volatile memory (any type of randomaccess memory), a non-volatile memory such as a flash memory, and/orphase change memory cells 10.

The I/O device 520 may be utilized to generate a message. The system 500may use the wireless interface 540 to transmit and receive messages toand from a wireless communication network with a wireless radiofrequency (RF) signal. Examples of the wireless interface 540 mayinclude an antenna or a wireless transceiver, such as a dipole antenna,although the scope of the present invention is not limited in thisrespect.

While the present invention has been described with respect to a limitednumber of embodiments, those skilled in the art will appreciate numerousmodifications and variations therefrom. It is intended that the appendedclaims cover all such modifications and variations as fall within thetrue spirit and scope of this present invention.

1. A method comprising: forming a substantially planar surface; andforming a phase change material sandwiched between a pair ofhorizontally spaced electrodes formed on said substantially planarsurface; forming two pairs of electrodes for two spaced cells at thesame time; and depositing a material to form said electrodes in atrench.
 2. The method of claim 1 including enabling light to access saidphase change material.
 3. The method of claim 1 including forming aconductive line in a substrate and forming said material and saidelectrodes over said substrate.
 4. The method of claim 3 includingforming a selection device in said substrate.
 5. The method of claim 4including forming a electrical connection from said substrate to asecond electrode.
 6. The method of claim 5 including electricallycoupling said second electrode to one of said horizontally displacedelectrodes.
 7. The method of claim 1 including covering at least aportion of said phase change material with an optically transmissivematerial.
 8. The method of claim 1 including clearing the bottom of thetrench to separate said electrodes and filling the remaining portion ofsaid trench with the phase change material.
 9. The method of claim 1including covering said phase change material with a light transmissivematerial.